The invention relates to spatial light modulators, and more particularly to improvements therein which result in low voltage, high speed devices with high contrast ratios.
PLZT (lanthanum modified lead zirconium titanate) is well known for use in spatial light modulators (SLMs), or “light valves”. U.S. Pat. No. 5,198,920 by the present inventor gives general background on spatial light modulators. Also see the article “Photoconductive Activated Light Valve for High Definition Projection System”, by Garth Gobeli and Thomas Toor, 172/SPIE vol. 1664 High-Resolution Displays and Projection Systems (1992).
FIG. 1 shows a prior art spatial light modulator formed in a relatively thick (e.g., 200 microns) substrate 10 of PLZT. On the top surface 15A there are formed various positive electrodes, for example, positive electrodes 11 and 13, and various negative electrodes, such as negative electrodes 12 and 14, interspaced between the positive electrodes. The opposite voltages applied to the positive and negative electrodes produce “fringing” electric fields 15 between the positive and negative electrodes underneath upper surface 15 of PLZT substrate 10. The presence or absence of the fringing fields 15 modulate or control the amount of light that can pass through the spatial light modulator or light valve 1.
A substantial problem of the above prior art of FIG. 1 is that the fringing electric fields 15 are relatively nonuniform. This non-uniformity results in the necessity of applying large voltage differences, usually more than 60 volts, between the positive and negative electrodes in order to achieve the desired level of light modulation. It would be very desirable to be able to use lower electrode voltage differences of less than 60 volts. The non-uniformity of the fringing electric field also means that memory mode material cannot be employed as a substrate.
Very thin layers of PLZT material of thickness in the range from 0.01 to 0.08 microns have been fabricated by depositing PLZT material onto a suitable substrate, using sputtering or liquid phase deposition techniques. Such very thin PLZT films require high activation voltages which lie well to the left of the minimum “A” shown in the curve of “PLZT operating voltage versus PLZT layer thickness” shown in subsequently described FIG. 8. Also, PLZT substrates with thicknesses in the 100 to 200 micron range have been fabricated by conventional grinding and polishing techniques, but the substrates of this thickness range require very high operating voltages that lie far to the right of the minimum “PLZT operating voltage versus PLZT layer thickness” shown in the curve of FIG. 8.
There would be a great many applications in the fields of optical computing, optical projectors, and large dynamic range cameras, for a two-dimensional spatial light modulator array in which each spatial light modulator cell operates with voltages less than approximately 60 volts, at operating speeds of more than approximately one million operations per second, and with a contrast ratio of greater than approximately 512 to 1.
Until now, no available or proposed spatial light modulator has been capable of meeting all three of these operating objectives. For example, typical liquid crystal devices (LCDs) operate at low voltages (less than 5 volts) but are quite slow, typically switching at about 30 frames per second, and have a low contrast ratio, typically about 12 to 1.